On the alignment of haloes, filaments and magnetic fields in the simulated cosmic web

TL;DR

This study uses cosmological magnetohydrodynamical simulations to analyze the alignment of magnetic fields, haloes, and filaments in the cosmic web, revealing biases in magnetic field measurements due to filamentary alignment.

Contribution

It introduces a new algorithm to study 3D filamentary connectivity and explores how physical processes influence magnetic field alignment in the cosmic web.

Findings

Magnetic fields tend to align with filaments, affecting measurement accuracy.

Alignment degree depends on physical processes and initial conditions.

Magnetic field estimates from Faraday rotation can be underestimated by a factor of 3.

Abstract

The continuous flow of gas and dark matter across scales in the cosmic web can generate correlated dynamical properties of haloes and filaments (and the magnetic fields they contain). With this work, we study the halo spin properties and orientation with respect to filaments, and the morphology of the magnetic field around these objects, for haloes with masses in the range 1e8-1e14 Msun and filaments up to 8 Mpc long. Furthermore, we study how these properties vary in presence, or lack thereof, of different (astro)physical processes and with different magnetic initial conditions. We perform cosmological magnetohydrodynamical simulations with the Eulerian code Enzo and we develop a simple and robust algorithm to study the filamentary connectivity of haloes in three dimensions. We investigate the morphological and magnetic properties and focus on the alignment of the magnetic field along filaments: our analysis suggests that the degree of this alignment is partially dependent on the physical processes involved, as well as on magnetic initial conditions. We discuss the contribution of this effect on a potential attempt to detect the magnetic field surrounding these objects: we find that it introduces a bias in the estimation of the magnetic field from Faraday rotation measure techniques. Specifically, given the strong tendency we find for extragalactic magnetic fields to align with the filaments axis, the value of the magnetic field can be underestimated by a factor 3, because this effect contributes to making the line-of-sight magnetic field (for filaments in the plane of the sky) much smaller than the total one.